1. Field
The disclosed embodiments relate to touch screens.
2. Background
Many electronic devices such as, for example, cellular telephones have touch screens (sometimes referred to as “touch panels”). By using a touch screen, the display area of the electronic device serves both as a display and also as a user input interface to enable a user to interact with and control the electronic device.
FIG. 1 (Prior Art) is a conceptual diagram of one type of touch screen 1. Touch screen 1 involves a first sheet 2 of transparent resistive material and a second sheet 3 of transparent resistive material. These two sheets are disposed over the display of the electronic device so that the display can be seen by the user through the touch screen. A first conductive bus bar 4 is attached to the upper left edge of sheet 2 and a second conductive bus bar 5 is attached to the lower right of sheet 2. Similarly, a third conductive bus bar 6 is attached to the upper right edge of sheet 3 and a fourth conductive bus bar 7 is attached to the lower left edge of sheet 3. When the touch screen is not being touched, the two sheets 2 and 3 do not touch one another. When the touch screen is pressed at a point of contact, the pressure of the touching causes the two sheets 2 and 3 to make electrical contact with one another at the point of contact. Electronics coupled to the touch screen determines an X-coordinate and a Y-coordinate on the touch screen that indicates the point of contact.
FIGS. 2 and 3 (Prior Art) are conceptual schematic diagrams that illustrate how the touch screen and its associated electronics determine the X-coordinate and the Y-coordinate of the point of contact. FIG. 2 is a cross-sectional side view of the touch screen. The upper row of resistors represents the upper sheet 2. The lower row of resistors represents the lower sheet 3. FIG. 2 illustrates the touch screen when the user is not touching the screen and the two sheets 2 and 3 are not touching each other. At a first time, a voltage is impressed between XP_UL and XM_LR. The YM_LL end of sheet 3 is made to be an open, and a high input impedance voltage sensor 8 is used to detect a voltage on sheet 3. In the case of FIG. 2, the lower sheet 3 does not receive a voltage from upper sheet 2 and this condition is sensed by sensor 8. At a second time, a voltage is impressed between YP_UR and YM_LL. The XM_LR end of sheet 3 is made to be an open, and a high input impedance voltage sensor 9 is used to detect a voltage on upper sheet 2. In the case of FIG. 2, upper sheet 2 does not receive a voltage from lower sheet 3 and this condition is sensed by sensor 9. From the voltages detected by sensors 8 and 9 at the first time and second time, the electronics of the touch screen determines that the two sheets 2 and 3 are not touching each other.
FIG. 3 (Prior Art) illustrates the touch screen when the user is touching the screen. The two sheets 2 and 3 are therefore touching each other at a point of contact as illustrated. At a first time, a voltage is impressed between XP_UL and XM_LR. The YM_LL end of sheet 3 is made to be an open, and sensor 8 is used to detect a voltage on lower sheet 3. The upper sheet 2 forms a resistive voltage divider with the point of contact being a tap on the voltage divider. There is no current flow through lower sheet 3 due to YM_LL being open and due to sensor 8 being a high input sensor. The voltage sensed by sensor 8 is therefore the voltage on the tap of the voltage divider. The magnitude of the sensed voltage therefore indicates the location of the touching between XP_UL and XM_LR. The voltage may be converted into a digital value and this digital value may be considered to be the X-coordinate of the point of contact. Then, at a second time, a voltage is impressed between YP_UR and YM_LL of the lower sheet 3. The XM_LR end of upper sheet 2 is made to be an open, and high input impedance voltage sensor 9 is used to detect a voltage on sheet 2. The lower sheet 3 forms a voltage divider with the point of contact being a tap on the voltage divider. There is no current flow through upper sheet 2, so the voltage sensed by sensor 9 is the voltage on the tap of the voltage divider, and therefore indicates the location of the touching between YP_UR and YM_LL. This voltage may be converted into a digital value and this digital value may be considered to be Y-coordinate of the point of contact.
FIG. 4 (Prior Art) is a simplified diagram of one type of conventional touch screen controller integrated circuit 10. At a first time, control portion 11 causes switches 12 and 13 to close such that a regulated analog supply voltage AVDD is supplied onto terminal 14 and such that terminal 15 is grounded. The voltage AVDD is therefore supplied across sheet 2. Analog multiplexer 16 is controlled such that the voltage on terminal 17 is supplied onto an input of an analog-to-digital converter (ADC) 18. ADC 18 converts the voltage on terminal 17 into a multi-bit digital value usable as the X-coordinate. At a second time, control portion 11 causes switches 19 and 20 to close such that voltage AVDD is supplied onto terminal 17 and such that terminal 21 is grounded. The voltage AVDD is therefore supplied across sheet 3. Analog multiplexer 16 is controlled such that the voltage on terminal 14 is supplied onto the input of ADC 18. ADC 18 converts the voltage on terminal 17 into a multi-bit digital value usable as the Y-coordinate. Battery voltage VBATT between terminals 22 and 23 is regulated to generate the analog supply voltage AVDD. Rather than there being two sensors 8 and 9 as illustrated in the conceptual diagrams of FIGS. 2 and 3, the functions of sensors 8 and 9 are performed by multiplexer 16 and ADC 18 in FIG. 4.
The touch screen is usable in different situations where different amounts of precision of detecting the point of contact are required. If, for example, large selectable icons may be displayed on the screen. If this is the case, then the detection of the point of contact need not be very precise in order for the electronics of the cellular telephone to determine that a particular large icon is being pressed. In such a situation, ADC 18 can be controlled via bus 24 and register 25 to operate as a lower resolution ADC that outputs multi-bit digital values of a smaller number of bits. If, however, the screen is to be used to detect the selection of very small icons or to detect a user writing on the screen (a user may, for example, use a fine tip stylus to write on the screen), then the detection of the point of contact should be more precise. In this situation, ADC 18 may be controlled to operate as a higher resolution ADC that outputs multi-bit digital values of a larger number of bits. Touch screen control circuitry such as that illustrated in FIG. 4 is sometimes embodied in digital baseband integrated circuits within cellular telephones.